noted in the last post, he also found a series of unusual "thrusts" that were distinguished by having younger rock on top of older sequences. A nearly horizontal young rock-on-old rock fault can be seen crossing the middle of the photo above.
Another geologist, H. D. Curry, who worked the region in the late 1930s noticed some very strange structures in the Black Mountains on the east side of Death Valley. He called the huge dome-like mountains turtleback faults. That's the Copper Canyon turtleback in the picture at the top of this post. The turtlebacks (there are three obvious ones in the Black Mountains, and a number of somewhat cryptic ones scattered through other mountain ranges across the region) are made of ancient Proterozoic-aged metamorphic rock, and their surfaces are fault planes. When rocks are found above the fault planes, they are invariably younger. The enigmatic young-on-old 'thrusts' and turtlebacks seemed to be closely related.
Other geologists started to question the idea that these faults were compressional. Even though Noble was initially sure that the faults were thrusts, over the years he began to accept that his first perceptions might possibly be invalid. The 'thrusts" were associated with numerous normal faults, and it was known by then that the Basin and Range province was produced by extensional forces in middle and late Cenozoic time. Finding the odd exception to the rule is not unusual, but by the 1970s, geologists were finding these enigmatic fault systems all across the Basin and Range province. As they realized the extensional nature of these faults, they knew they had discovered a counterpart to the compressional thrust faults. These low-angle normal faults were soon being called detachment faults, and the dome-like systems of ancient metamorphic rocks were termed metamorphic core complexes.
this report by Miller and Pavlis for a review of the issues involved, or pick up the excellent guide to the geology of Death Valley by Miller and Wright.
|Diagram of a 'typical' detachment fault from Spencer and Reynolds, 1989, Middle Tertiary Tectonics of Arizona |
and Adjacent Areas, Arizona Geological Society Digest 17, p. 539-574.
When one stands on the top of the Black Mountains and looks across the incredible Death Valley graben to the Panamint Mountains on the far side, one can barely comprehend the implication of the discovery of detachment faulting and metamorphic core complexes. The Panamints were once on top of the Black Mountains, or nearly so. And only a short time ago in the geologic sense, just a few million years before the present.